FOR THE CAKE
Cooking spray
1 1/2 c. all-purpose flour
1 1/2 tsp. baking powder
1/2 tsp. kosher salt
5 large eggs, separated
1 1/4 c. granulated sugar, divided
1 tsp. pure vanilla extract
1/2 c. milk
4 tbsp. melted butter, cooled
FOR THE COFFEE MIXTURE
1/2 c. strong brewed coffee
1 tbsp. dark rum
FOR THE FROSTING
1 1/2 c. (3 sticks) butter, softened
8 oz. mascarpone, room temperature
1 tsp. pure vanilla extract
1/2 tsp. kosher salt
3 c. powdered sugar
1/4 c. heavy cream
2 tbsp. cocoa powder, for garnish
Step 1
Make the cake: Preheat oven to 350° and grease two 8" pans with cooking spray and line with parchment. In a large bowl whisk together flour, baking powder, and salt.
Step 2
In another large bowl using a hand mixer, beat together egg yolks and 1 cup sugar until stiff ribbons trail behind beaters, then beat in vanilla and set aside.
Step 3
In a third large bowl using a hand mixer, beat egg whites until soft peaks form then gradually add in remaining 1/4 cup sugar and continue beating until stiff peaks form.
Step 4
Add yolk mixture to dry ingredients and beat together. Add in milk and melted butter and beat until combined. Gently fold in egg whites until just combined.
Step 5
Pour batter into prepared pans and bake until a toothpick inserted into middle of the cakes comes out clean and tops are lightly golden, 26 to 28 minutes.
Step 6
Let cool completely, then slice in half to create 4 thin round layers.
Step 7
In a small bowl or measuring cup, stir to combine coffee and rum.
Step 8
Make frosting: In a large bowl using a hand mixer or the bowl of a stand mixer using the paddle attachment, beat butter, mascarpone, vanilla, and salt until smooth. Add powdered sugar gradually, until completely combined. If buttercream is too thick, add heavy cream one tablespoon at a time until preferred texture is reached.
Step 9
Build cake: Place first layer of cake on serving dish, then brush cut side with coffee mixture. Add a layer of frosting to the top, then top with next cake layer. Continue brushing layers with coffee mixture and topping with an even layer of frosting until all layers are used. Frost sides of cake, reserving remaining frosting for the top of the cake.
Step 10
Transfer remaining frosting to piping bag, then pipe dollops all over the top of the cake. Dust with cocoa powder before serving.